Everyone knows that if you want to avoid getting kicked you can open up a trade menu with NPC or keep stash open.

If he's AFKing (outside of town) to avoid getting kicked, that means he's taking into account the fact that people might notice his being AFK. If people notice that you're AFK and they can't kick you, the next logical step is obviously training a group of mobs on you to kill you.

Let's assume that he did not know how to AFK in town without getting kicked, and he just wanted to leech bounty rewards. Then his logic would be: "I will AFK outside of town for a reward of a few horadric caches." The risk is categorically NOT worth the risk of permanent death. No rational person (not even a stupid person) would want to risk that much for so little.

Thus, there is ZERO incentive to AFK at a wp (from the perspective of leeching), so it's likely that an emergency came up.

Edit: I can understand the OP's frustration and his quick conclusion to kill the AFKer, but if you sit down and really think about it, the conclusion is that it's highly likely that the AFK player wasn't trying to leech off his team mates.